The present invention relates to non-impact printers employing an exposure device such as a printhead with a plurality of recording elements emitting light for recording latent images on a photosensitive recording member, such as an organic photoconductive (OPC) belt. Such a printer can be of the type using an exposure device with an array of recording sources such as light emitting diodes (LEDs). Lens means such as a rod lens array (commercially available under the trade-marked name SELFOC) can be used for focussing the light emitted by the LEDs on the photosensitive recording member. Printers of the above-mentioned type also comprise developing means for developing the latent image formed on the photosensitive member into a visual toner powder image. Such printers further comprise transfer means for transferring the toner powder image from the photosensitive recording member onto an image receiving medium such as a sheet of paper.
In printers of the above-mentioned type, the LEDs are mounted on a solid substrate and generally arranged in rows across the width of the photosensitive recording member. LEDs can be integrated on LED chips, each one of the chips containing, for example, a block of 128 integrated LEDs. A number of LED chips can be mounted on a module plate and several module plates can be mounted such that a print bar is formed whereon LEDs are spaced with a constant pitch.
Energy output levels are applied to the LEDs by associated drivers, in order to produce light spots on the photosensitive receiving member for producing an image made of picture elements (pixels). Spots having multiple energy levels are obtained by providing multiple levels of output power for a constant period of time, or by providing a constant output power level for a period of time proportional to the gradation value of a pixel. In so-called binary printers, only two possible energy levels can be applied to a LED, one level for giving rise to a light spot, the other level being a zero energy level. If a charge area development process is used, a light spot projected on the photosensitive member with a light intensity larger than a so-called print threshold intensity is discharging locally the photosensitive material and no toner is developed locally (no pixel). If a charge area development is used and a LED is not driven (zero-energy level), the photosensitive member remains locally charged and toner is locally transferred for giving rise to a pixel.
The unevenness of the optical density in images obtained with printers using such an exposure device comprising LEDs has to be minimised. The evenness of the optical density of a printed image is related to the features of the exposure intensity profile on the photosensitive member when LEDs are activated. Let us define the uniformity degree of an exposure intensity profile as the degree of regularity of the profile considered at the print threshold intensity when the LEDs are driven according to a regular scheme, for example, when pairs of LEDs in a row are driven alternatively. An important attribute of the exposure intensity profile on the photosensitive member is its uniformity degree because it directly relates to the evenness of the printed images. To reduce the unevenness of the optical density of printed images, the uniformity degree of an exposure intensity profile should be high. An exposure intensity profile with a low uniformity degree is caused, for example, by differing light intensity of the LEDs due to production process or material, temperature dependence of the LED light output yield and differing light transparency of the lens means (for example, a Selfoc lens array) across the print width. Another source for an exposure intensity profile with a low uniformity degree are local imperfections of the rod lens array, such as anomalous lens rod fibers or misaligned lens rod fibers. An exposure intensity profile with a low uniformity degree can also be caused by height differences of LEDs, or of LED-chips or of chip module plates.
An exposure device with an integrated alignment system for aligning the individual light intensities emitted by the LEDs to an equal intensity level is disclosed in the book entitled ‘The World of Printers’ published by Océ Printing Systems GmbH, edited by G. Goldmann, Mai 2001, under ISBN 3-00-001019-X. On page 13 of chapter 5 of said document, it is disclosed how to align the intensity distribution to ensure equal light intensity for all LEDs of a LED print bar. The various light intensities of the individual LEDs are measured by a photosensor mounted on a motor-driven guide block and being moved across the print width. From the light intensities measurements, corrective factors for the current values to be applied by the drivers to the LEDs are derived and stored in a memory.